”I’m keeping my fingers crossed,” director Joe Johnston mutters, and well he might. It’s 1 a.m. on a windswept airfield in Novato, Calif., and at the moment the success of the gaudiest special-effects sequence in a movie crammed with such spectacles literally hangs in the balance before his eyes. The Rocketeer, Walt Disney Pictures’ breathless Art Moderne valentine to old-fashioned, golly-gee, whiz-bang Saturday matinees, is designed to soar from one high-octane technical marvel to another as it spins its fanciful tale: All-American fly-boy discovers a top secret rocket pack that pits him against a murderous web of mobsters, G-men, and Nazis. Turning the rich panels of Dave Stevens’ cult comic book, on which the movie is based, into a seamlessly realistic film has proved to be a Herculean task. And on this night director Johnston is going to risk $400,000 of his budget on one special effect.

He’s about to film the climactic battle aboard a Nazi zeppelin hovering high above Los Angeles’ Griffith Observatory, the movie’s eye-popping finale. In the film, which opened June 21, 1991, the scene plays as one great nonstop whoosh of adrenaline-charged excitement. The Rocketeer (played by Bill Campbell) swoops down on the zeppelin to save his lady love, Jenny (actress Jennifer Connelly, now Campbell’s offscreen fiancée as well) from the dastardly clutches of Neville Sinclair (Timothy Dalton), a dashing Hollywood actor who’s also a Nazi spy. In reality, the sequence was filmed piecemeal over more than four months. The ground action at the observatory, with Sinclair abducting Jenny and flying off in the zeppelin, was shot over the course of several chilly nights back in November 1990. The company then moved on to Indian Dunes Raceway, 45 minutes north of L.A., where the actors battled it out atop a 65-foot section of the zeppelin’s hull. Meanwhile, at the wizardly Industrial Light & Magic factory in San Rafael, model makers filmed a 12-foot miniature of the entire zeppelin against re-creations of the Los Angeles basin, circa 1938, for intercutting.

Now, camped out on this March night at Hamilton Air Force Base just north of ILM headquarters, the filmmakers are dearly hoping they can successfully capture the pièce de résistance of the elaborate sequence, a shot in which the zeppelin bursts into extravagant flame and falls, with luck gracefully, to earth. Hanging from scaffolding suspended between two 15-foot cranes and swaying uneasily in the breeze is the star of the moment, a delicate, 35-foot model of the zeppelin, detailed down to the bullet-shattered windows of its tiny gon-dola. Five cameras have been positioned along a huge arc to capture the zeppelin’s demise at 350 frames per second; when the shot is projected back at the standard 24 frames per second, the model will look like an 850-foot airship going down in a formidable fire storm.

Tonight the film’s met-on-the-set couple, Campbell and Connelly, are not on hand (”I feel good about their relationship,” says Johnston. ”It makes their relationship on-screen that much more believable”). In fact, none of the actors is present, having completed the film’s 96 days of principal photography: The human center of attention is visual effects supervisor Ken Ralston, who knows well what is riding on his performance.

”Am I tense?” Ralston says. ”Yes. When this thing starts, no matter how much planning you put into it, it’s all ad-lib.” Since it’s part of his job to trigger the radio-controlled explosives that have been hidden beneath the zeppelin’s skin, he adds, ”I have to stay calm. I have to make sure I don’t get caught up in the excitement of the moment.”

While the crew sees to the last-minute details — wiring the explosives, touching up the paint job-things are anything but exciting: The short shot requires hours to prepare. Stalking the set, calm but weary, Johnston and Ralston have almost reached the end of the movie’s painstaking special-effects schedule.

Hard as the zeppelin sequence has been, many pains came in reproducing the ’30s California, streamlined style favored by cartoonist Stevens, who first created The Rocketeer in 1982. ”I’d been a ’30s fiend for a long time,” says Stevens. ”I loved the music, the clothing, the cars, the look.” And especially the gimmicky, larger-than-life architectural jokes that once could be found in Southern California. ”During the Depression,” Stevens recalls, ”my dad’s family lived in the bottom half of a giant ice cream cone. It probably spurred me to create the Bull Dog Cafe.” Needless to say, the Bull Dog, the Rocketeer’s favorite hangout, has the shape of a giant pooch.

The price of re-creating L.A. in all its 1938 glory, then piling on plenty of airborne special effects, was high. In his famous budget-cutting memo, Disney Studios chairman Jeffrey Katzenberg praised The Rocketeer as an example of the studio’s new cost-consciousness. But even as he was citing the movie as a calculated $35 million gamble, its budget was rising, eventually reaching the $40 million mark. Actually, says Johnston, ”We agreed on $25 million, but once [Disney] started seeing footage, they realized this was a bigger movie than they were anticipating, and they approved overages. The budget kept climbing, but it never got completely out of control.”

On the other hand, risking $400,000 on a climactic effect that fails would not exactly add to The Rocketeer’s standing as a model of cost efficiency.

The clock ticks on toward 2 a.m., and the final countdown begins — and is immediately halted. There’s a problem with the remote controls. The lights are doused and the explosives temporarily defused. Johnston still seems confident. Before directing (Honey, I Shrunk the Kids), he worked at ILM, serving as an art director on the Star Wars and Raiders series. ”I’ve worked with these guys enough to know they can do just about anything,” he says.

The lights are fired up. The cameras are ordered to begin rolling.

”HIT IT!” commands Ralston, and Johnston hunches over the monitors.

In seconds, the zeppelin explodes in huge, orange flames and, like a great, fiery whale, flops awkwardly to the ground.

Johnston and Ralston silently watch the monitors as the action is replayed. They won’t know for sure how it went until they see the developed film the next day, but their gut instinct tells them it didn’t work, the explosives having apparently misfired. Instead of the graceful swan dive they’d hoped for, the zeppelin’s fall looks more like a belly flop. Neither speaks as they play back the disappointing tape again and again.

Six weeks later, the ILM team constructed a new zeppelin. This time the radio control devices functioned properly and the explosives were all triggered in the appropriate order. The zeppelin crashed and burned in a massive display of pyrotechnics.